So they're girls and they change clothes...I'm all right with it!
It's true, though, Yuna's change from a somber priestess in FFX to a gun-wielding pop singer was a bit jarring at first, but...knowing what kind of profound impact Tidus had on Yuna in the first game makes her attitude adjustment seem to make sense. Besides which, the new flow of the game, making Yuna and her friends a team that travels the world solving peoples' problems and sadnesses, makes additional sense...Yuna and her group of Gullwings seek to go wherever they please (or wherever the player pleases) and bring joy to those who need help. Giving the player those choices and that freedom is a great twist. Normally, FF is a one way journey...there's occasional deviations available along the way, but allowing the player in FFX-2 to go wherever they want, whenever they want, makes for loads of possibilities.
Ultimately, was this game truly necessary to have? Was it truly and utterly needed to follow Yuna, Rikku and a new chica as they traipse around the world? Probably not. After all, it wouldn't be the first time a Final Fantasy game had a very balanced ending...extremely hopeful for the future, yet immensely heart-wrenching. But...Square-Enix insisted, and they certainly succeeded, could've been a hell of a lot worse, that's for sure.